This invention relates to a composition of solder suitable for low temperature soldering and rework of printed circuit boards.
More specifically, this invention relates to a composition of low temperature solder comprising tin, in which is also present a dopant level of copper in an amount below the binary tin-copper eutectic point. The new solder may be applied to the material to be soldered using solder fountain and other conventional soldering techniques. Because of its low melting point relative to other copper containing solders, the new solder may be used for nondestructive soldering of materials not amenable to being soldered by copper containing solders available heretofore.
Most specifically, this invention relates to a composition of copper doped low temperature solder suitable for use in component assembly and rework on printed circuit cards or boards.
There has been a wide range of solder compositions reported in the art. By a judicious choice of elemental ingredients and proportions, solders have been devised for a number of specific applications, such as to wet and bond to carbon and semiconductors (U.S. Pat. No. 4,512,950 to Hosoda et al, issued Apr. 23, 1985 and U.S. Pat. No. 4,396,677 to Intrater, issued Aug. 2, 1983), to inhibit the formation of oxide (U.S. Pat. No. 4,654,275 to Bose, issued Mar.31, 1987, to increase tensile strength (U.S. Pat. No. 4,588,657 to Kujas, issued May 13, 1986), to soft solder aluminum (U.S. Pat. No. 4,070,192 to Arbib et al, issued Jan. 24, 1978, to bond semiconductor to metal (U.S. Pat. No. 4,357,162 to Guan, issued Nov. 2, 1982), to bond oxide surfaces using vibration (U.S. Pat. No. 4,106,930 to Nomaki et al, issued Aug. 15, 1978), to solder joints for lead-free potable water (U.S. Pat. No. 4,695,428 to Ballentine et al, issued Sept. 22, 1987), and to improve the tarnish resistance of jewelry (U.S. Pat. No. 3,650,735 to Monaco, issued Mar. 21, 1972).
However, the problem which is solved by the present invention is quite different: how to assemble and rework a through hole connection in a printed circuit device in such a manner that the dissolution of a copper coating on the walls of the through hole connection into the solder is suppressed. Rework is reflowing the solder in the joint. The present invention is directed to a novel solder composition which retards the dissolution of copper within and surrounding a through hole or blind via and operates at a lower temperature than before possible with available copper containing solders, permitting assembly and repeated rework of faulty solder connections in and around the through holes of printed circuit boards while also avoiding other problems associated with higher temperature rework, such as delamination of layers in the laminate board, localized mechanical distortion, and degeneration of solder joints and exceeding the critical temperature of the mounted device.
In U.S. Pat. No. 2,671,844 to Laubmeyer et al, issued Mar. 9, 1954, it is recognized that the addition of an amount of copper to a tin/lead solder slows the wear of an electrical soldering bit made of copper, which tended to dissolve in the solder. The addition of 3% copper to the solder reduced the rate of dissolution of the copper bit by 25%, and 5% copper reduced the rate to 10% of the rate it was with no copper present in the solder. Any effect on soldering temperature as a result of the addition of the copper to the solder was not described, and this technique became obsolete with the advent of nonreactive bits and non-contact soldering methods. The '844 patent described that it was "essential that an eutectic of tin and copper should form in the (solder) alloy", that eutectic consisting of 99% Sn and 1% Cu (see column 2, lines 30-37). It also described that whatever the amount of copper in the solder, it should be minimally sufficient to form this binary tin-copper eutectic point. Thus in a 50/50 Pb/Sn solder, Cu in the amount of 0.5% of the total weight of the solder should be added.
In U.S. Pat. No. 4,622,205 to Fouts et al, issued Nov. 11, 1986, copper in Pb/Sn solder in a certain amount extended the life of copper conductor lines by decreasing electromigration between the solder joint and the line during the passage of current at an operating range of 50.degree. C.-90.degree. C. The source of the difficulty with respect to electromigration was attributed to non-uniform distribution of intermetallic particles of Cu.sub.3 Sn and trace Cu.sub.6 Sn.sub.5 in the solder. The solution to the problem was identified as adding to the solder an element which forms an intermetallic alloy with tin, such as copper, in an amount at least about 0.5% to less than about 10% of the solder, preferably 3.5 wt. % Cu, to slow grain boundary diffusion, thereby reducing the tendency to electromigration and thermomigration.
Copper containing solder has been obtainable on the marketplace since at least about 1964, the publication date of Solders and Soldering, pp. 65-66 McGraw-Hill Book Co., H. H. Manko. However, this reference does not give specific compositions of commercially available copper containing solders, saying only that the amount is "high".
A commercially available product called Savbit 1 alloy, obtainable from Multicore of Westbury, NY, comprising about 1.5% to about 2.0% Cu by weight, has been used to extend the life of copper soldering tips in static hand soldering applications. Product data sheets, however, recommend a minimum bit temperature of 272.degree.-294.degree. C., above the range required for nondestructive assemblies and rework on printed circuit boards and cards.
Other problems which can result from the addition of copper to solders include increase in solidification time and formation of bridges (shorts), cold solder joints, nodules and various additional defects.